There is a seal and O-ring on each side. They both need replaced. Either can cause a leak. you will have to dissassemble the stub axle assembly to replace the seal. Not a big deal.
Pull up the ERA owners only Jag Rear Suspension Service. Item #34 on the parts exploded view is the seal the can't be seen in your photo.
Be certain you have the correct vintage seal. A later seal in the earlier differential will leak. Ask me how I know.
No need for sealant on the surfaces you show. The hole shown will not drip
oil if your seals are good.
John